1. Field of the Invention
The present invention relates to a system and method for controlling ignition timing for an internal combustion engine and particularly relates to the system and method for controlling the ignition timing applicable to vehicular engines which effectively avoid occurrence of engine knocking during a high engine load and thus improve engine performance.
2. Description of the Prior Art
Ignition timing for an internal combustion engine must be determined in relation to engine operating conditions so that the engine is operated optimally.
In general, taking efficiency and fuel economy of the engine into account, it is most preferable to ignite each of the engine cylinders at a timing in the vicinity of a minimum advance angle in order to generate maximum engine torque, i.e., to provide so called MBT (Minimum advance for Best Torque). Therefore, it is necessary to change the ignition timing to the MBT state according to the engine operating conditions.
However, since engine knocking tends to occur under certain engine operating conditions when the ignition timing is advanced, a stable engine operation cannot be achieved. For example, engine knocking tends to occur under transient engine operating conditions.
One ignition timing control system for vehicular engines which prevents engine knocking during engine transient operating states is disclosed in Japanese Patent Application Unexamined Open No. Sho 60-26170 published on Feb. 9, 1985.
In the above-identified Japanese Patent Application document, the control system continually checks to see whether the engine is accelerating and when the engine is accelerating, it reads out one of a plurality of retardation angle correction amounts preallocated according to the instantaneous throttle valve opening angle and according to the rate of change of the throttle valve opening angle. A basic ignition timing value determined according to engine operating conditions is corrected on the basis of the preallocated retardation angle correction amount. In this way, the engine knocking can be avoided by using the resulting ignition timing angle as the actual ignition timing.
In this conventional ignition timing control system, the basic ignition timing is corrected on the basis of a preallocated retardation angle correction amount during engine acceleration. Therefore, in a case, e.g., when an acceleration change (a gear change with a throttle valve opening angle not changed) is carried out in an automatic transmission (AT) system of the vehicle, the engine becomes a high load state due to air introduction through an inertial force thereof toward an intake air passage of the engine and an abrupt change of an engine speed. However, an immediate correction of the ignition timing toward the retardation angle side cannot be carried out to cope with such an abrupt transfer to a high engine load state so that the engine knocking will result.